A common reason for failure of drugs in development or their removal from the market is hepatotoxicity. This is due to the many adverse drug reactions and drug-drug interactions that result from drug metabolism, enzyme induction or inhibition of drug metabolizing enzymes. Unfortunately, effective in vitro and in vivo approaches to accurately predict hepatotoxicity of new chemical entities in humans are lacking. Numerous reports have shown recently that changes in gene activity can provide highly valuable information about toxicological endpoints. Within that area, RNA profiling has typically been the standard for identifying differential gene expression. However, these methods also have limitations. A new and novel platform technology, called TranscriptionPath and based on chromatin immunoprecipitation of DNA-transcription machinery complexes, identifies genes undergoing active transcription inside cells and quantifies very accurately and reproducibly their levels of transcription. Further, TranscriptionPath is powerful in being able to measure very early transcriptional changes inside cells and tissues, which should permit the identification of changes at the beginnings of metabolic pathways. This proposal will apply the new TranscriptionPath method to the identification of gene transcription changes in liver at early time points after treatment of rats with drugs known to result in chemically-induced hepatotoxicity. In addition, the same drugs will be used to treat isolated rat hepatocytes, which will also be transcriptionally profiled to determine if the same gene expression changes are observed in vitro. Gene transcription changes will also be correlated with functional biochemical changes. In Phase II, the diagnostic profiles derived from the rat studies will be used to assess the cross-species scaling of the hepatotoxic endpoints and to identify new gene expression signatures predictive of hepatotoxic potential in humans. Development of the technology proposed herein should significantly help improve preclinical safety assessment and eventually help companies, governmental agencies and academic entities develop safer drugs in less time and with lower costs. [unreadable] [unreadable]